/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Find "new" group for input group underIndexmapping, or create new if missing
index of duplicate group
int FindOrCreateDuplicateGroup ( int TriangleID, [FMeshIndexMappings](API\Runtime\GeometryCore\DynamicMesh\FMeshIndexMappings) & IndexMaps, [FDynamicMeshEditResult](API\Runtime\GeometryCore\FDynamicMeshEditResult) & ResultOut )